(1). Field of the Invention
The present invention relates to a power transmission device for an internal-combustion engine for use in a motorcycle or the like, and in particular relates to an improvement of the primary driven gear meshing with the drive gear provided on the crankshaft.
(2). Description of the Prior Art
As is well known, motorcycles employ a power transmission device for transmitting the driving force of the internal-combustion engine to the drive shaft. This power transmission device is typically constructed so that the rotation of the crankshaft is transmitted through the drive gear that rotates together with this crankshaft, to a primary driven gear, and further this rotational power force is transmitted to the driven shaft in an appropriate manner, via a clutch mechanism provided on the side of the driven gear.
In such a power transmission device, there is a problem in that rotational irregularities of the crankshaft or the like causes a chatter or collision noise between the teeth of the primary driven gear and the drive gear, which is the main factor of high noise generation in the power transmission device. Since chatter is generated due to backlash formed between the aforementioned two gears, a conventional configuration has been proposed and implemented in which a scissor gear is additionally provided for the primary driven gear so that the aforementioned two gears will mesh with each other without producing any backlash. This scissor gear is of the same diameter and the same number of teeth as the primary driven gear, and is adapted to be arranged in a superimposed manner, on one side of the primary gear and is configured so that springs are inserted through openings formed in the two gears to continuously produce a constant phase difference between the two gears.
In the above case in which a scissor gear is provided for the primary driven gear, the geometrical arrangement of the scissor springs is very important. It is desired that the scissor springs be located as distant from the center as possible in view of reducing the spring size and strength and all the springs be arranged in a balanced manner.
However, the primary driven gear and scissor gear are formed with openings for damper springs that link the clutch mechanism, through-holes for engaging the clutch housing and the like. Accordingly, it has been very difficult to arrange the springs in a good enough manner to meet the above requirements.
There is also a conventional configuration using damper springs and a scissor gear (refer to Japanese Utility Model Publication Sho 61 No.23,968) in which some of multiple damper springs are used as the scissor springs. This configuration, however, suffers from a problem in that while the springs having both functions are functioning as the damper springs, the spring forces on the springs acting as the scissor springs vary, and thus the functionality of the scissor spring fluctuates.
A further conventional configuration using scissor springs and damper springs has been disclosed in Japanese Patent Application Laid-Open Sho 58 No.8,864, in which two types of rubber dampers are used in combination. However, this has a problem of low durability because of the rubber dampers.